This invention relates to a light emitting element array constructed by arraying a plurality of light emitting elements such as light emitting diodes in a row, an optical printer head using such a light emitting element array and a method for driving such an optical printer head.
FIG. 17 is a plan view showing a first conventional light emitting element array which is a semiconductor light emitting device used for the construction of an optical printer head, for example, as shown in Japanese Unexamined Patent Publication No. 61-205153. In FIG. 17, identified by 101, 103 and 113 are a light emitting element array as a semiconductor light emitting device, light emitting elements which are light emitting diodes (LEDs), and electrode pads, respectively. An array of the light emitting elements 103 are integrated at a density of 10 to 48 per mm. The electrode pads 113 are provided in one-to-one correspondence with the light emitting elements 103 and are connected with external circuits via bonding wires. Accordingly, power from a power supply is supplied to the light emitting elements 103 via the bonding wires.
In order to ensure a space sufficient to enable the connection of the bonding wires, the electrode pads 113 are arranged in an offset manner at the opposite sides of a substrate 1. For example, 64 to 256 light emitting elements 103 are monolithically formed per chip, thereby constructing one light emitting element array 101. An optical printer head is constructed by mounting such light emitting element arrays 101 on one or a plurality of circuit boards.
FIG. 18 is a perspective view of a first conventional optical printer head constructed using the aforementioned first conventional light emitting element arrays 101. In FIG. 18, identified by 101 are light emitting element arrays; by 110 a circuit board on which light emitting element arrays 101 are mounted; by 111 conductive patterns provided on the circuit board; by 112 bonding wires for connecting electrode pads 113 of the light emitting element arrays 101 and the conductive patterns 111 on the circuit board 110; by 120 a flexible printed circuit (FPC); by 119 drivers for driving the light emitting element arrays 101; and by 121 wiring portions from data input terminals of the drivers 119. The conductive patterns 111 on the circuit board 110 are substantially at the same pitches as the light emitting elements 103.
Such an optical printer head is assembled as follows. First, the light emitting element arrays 101 are mounted on and bonded to the circuit board 110 by die bonding and then the electrode pads 113 of the bonded light emitting element arrays 101 and the conductive patterns 111 on the circuit board 110 are connected by the bonding wires 112. On the other hand, output wires (extending toward the light emitting element arrays 101) of the drivers 119 connected with the FPC 120 by, e.g., inner lead bonding are connected with the conductive patterns 111 on the circuit board 110 by a laser or thermal adhesion. In this way, the light emitting elements 103 and the output wires of the drivers 119 have a one-to-one correspondence, and a current is supplied to the light emitting elements 103 via the bonding wires 112. Signals and voltages are supplied to the light emitting elements 103 and the drivers 119 via the wiring portion 121 of the FPC 120.
FIG. 19 is a perspective view of a second conventional optical printer head. In FIG. 19, identified by 101 a light emitting element array; by 112 bonding wires; by 119 drivers; and by 121 input wiring portions (input signal patterns) provided on a circuit board 110 to feed input signals to the drivers 119.
The optical printer head shown in FIG. 19 is assembled as follows. First, the light emitting element array 101 and the drivers 119 are mounted on and bonded to the circuit board 110 by, e.g., die bonding and then the electrode pads 113 of the light emitting element array 101 and output electrodes of the drivers 119 are directly connected in one-to-one correspondence by the bonding wires 112.
On the other hand, similar to the output electrodes, input electrodes of the drivers 119 are directly connected with the input signal patterns 121 on the circuit board 110 via the bonding wires 112.
A comparison of the aforementioned two conventional printer heads shows that they adopt different methods for connecting the drivers 119 and the light emitting element arrays 101. Specifically, in the first conventional optical printer head, the output wiring portions of the drivers 119 are connected with the light emitting element array 101 after being bonded to the conductive patterns 111 on the circuit board 110 as shown in FIG. 18. However, in the second conventional optical printer head, the drivers 119 and the light emitting element arrays 101 are directly bonded to each other as shown in FIG. 19.
Further, the first and second conventional optical printer heads are similar in that the drivers 119 provided on two chips are used for driving the light emitting element array 101 provided on one chip to emit a light.
Although the electrode pads 113 of the light emitting element array 101 are generally arranged in an offset manner along the arranging direction of the light emitting elements 103 as shown in FIG. 17, they may be formed only at one side of the light emitting elements 103 (one-side output method). In such a case, the light emitting element array 101 formed on one chip can be driven to emit light by the driver 119 formed on one chip.
However, in the first and second conventional optical printer heads, since fairly high bonding pitch precision is required, it has been difficult to improve productivity of the optical printer head.
Also, in the first and second conventional optical printer heads, half the number to the same number of drivers as a total number of the light emitting element arrays are provided. This necessitates a large space for mounting many drivers on the circuit board and a mounting process, which hinders a reduction of production cost of the optical printer head.
Further, since the light emitting element arrays and the drivers are provided in parallel in the first and second conventional optical printer heads, it has been difficult to narrow a width of the optical printer head along a sub-scanning direction, which stands as a large hindrance to making the optical printer head smaller.
Furthermore, in the first conventional semiconductor light emitting device used for the first and second conventional optical printer head, precision of the wire bonding process and a limit in narrowing the pitches of the electrode pads stand as a large hindrance to narrowing the pitches between the light emitting elements. For example, it makes it difficult to realize a pitch of 22 xcexcm or smaller between the light emitting elements which is required for 1200 dpi (dots/inch).
In view of this problem, a method for monolithically forming drivers on the light emitting element arrays 101 on which only the light emitting elements 103 and the electrode pads 113 were formed has been proposed to remarkably reduce the number of bonding as compared to the prior art, improve reliability, reduce production cost and enable high-quality printing resulting from the narrow-pitch-arrangement of the light emitting elements.
FIG. 20 is a plan view of a second conventional light emitting element array which is a semiconductor light emitting device used to construct an optical printer head, for example, as shown U.S. Pat. No. 4,587,717. Light emitting elements 103 which are GaP light emitting diodes, an output circuit 122a and a signal processing circuit 122b forming a driving circuit 122 for the light emitting elements 103 are monolithically formed on the same chip, i.e., a silicon substrate 102. Image data from the output circuit 122a are fed in parallel, serial or serial/parallel to the light emitting elements 103.
FIG. 21 is a plan view of a third conventional light emitting element array which is a semiconductor light emitting device used to construct an optical printer head (see Japanese Examined Patent Publication No. 6-94216). In FIG. 21, identified by 102 is a silicon substrate, on which a plurality of light emitting elements which are light emitting diodes and a plurality of driver elements 109 for driving these light emitting elements 103 are provided. The respective light emitting elements 103 and the respective driver elements 109 are monolithically integrated in one-to-one correspondence. In this way, the light emitting element array 101 including about 64 to 256 light emitting elements 103 and driver elements 109 is formed.
Identified by 124 in FIG. 21 is an element separating layer for optically separating the adjacent light emitting elements 103 and electrically separating the light emitting elements 103 and driver elements 109. The element separating layer 124 is provided around the light emitting elements 103 on the silicon substrate 102. A wiring portion 106 is adapted to electrically connect the light emitting elements 103 and the corresponding driver elements 109. Electrode pads 113 are formed on the silicon substrate 102, a number of electrode pads 113 necessary to feed signals for driving the light emitting elements 103 via the driver elements 109, e.g., 6 to 7 electrode pads 113 are provided for the light emitting element array 101 formed on one chip. Diffused resistors 108 form ohmic contacts with the light emitting elements 103 and provide electrical insulation from the silicon substrate 102 to restrict a current flowing between the light emitting elements 103 and the driver elements 109. It should be noted that an insulting layer 107 is provided on the driver elements 109.
FIG. 22 is an equivalent circuit diagram of the light emitting element array 101 shown in FIG. 21. Although this equivalent circuit diagram shows one light emitting element 103, the diffused resistor 108 which acts to restrict a current and the driver element 109 are connected with each light emitting element 103. In other words, the light emitting elements 103 are electively cause to emit light by the driver elements 109 and un-illustrated logic circuits (registers). In order to cause the light emitting element 103 to emit light, a constant voltage Vdd is applied to the driver element 109 after turning it on. At this time, a current flowing onto the light emitting element 103 is supplied via the diffused resistor 108.
FIG. 23 is a perspective view of a third conventional optical printer head constructed using a plurality of light emitting element arrays 101 shown in FIG. 21. Identified by 110 in FIG. 23 is a circuit board, on which a plurality of light emitting element arrays 101 are arrayed, mounted and fixed by an adhesive such that all the light emitting elements 103 are linearly aligned. Electrode pads of the light emitting element arrays 101 and a circuit pattern 111 (conductive patterns) formed on the circuit board 110 are connected via bonding wires 112. Signals from logic circuits and electric power are supplied to the respective light emitting element arrays 101 via an input connector 125 as an interface provided on the circuit board 110 to be electrically connected with the circuit pattern 111.
In the above-mentioned third conventional light emitting element array, however, the constant voltage Vdd is commonly applied to the respective light emitting elements 103 of the light emitting element array 101. In this case, the respective light emitting elements 103 are driven by controlling the transistors of the driver elements by the unillustrated logic circuit (shift register).
In this case, about 6 or 7 bonding pads are provided for feeding logic circuit signals and the constant voltage to drive the light emitting elements 103. However, because of driving by the constant voltage, it is difficult to correct an emission variation in the light emitting element array or among the light emitting elements.
Further, in the conventional light emitting element arrays, usually, the emission intensity of the light emitting elements 103 greatly varies in the light emitting element array 101 and among production lots of the light emitting element arrays 101. Such variations directly influence the printing quality of a printer using the optical printer head. Accordingly, light emitting element arrays in which the variation of the light emission is below xc2x110% are classified as good, while the other light emitting element arrays are classified and scrapped away as bad. Comparing to good light emitting element arrays, the number of bad light emitting element arrays is considerably great, consequently causing the problem that the yield of good light emitting element arrays is very low.
In view of the above, the conventional optical printer heads adopt a method for broadening a range of variation of emission intensity within which the light emitting element arrays are judged to be good products and improving a yield of the light emitting element arrays by providing the drivers for driving the light emitting element arrays with a function of correcting the variation of the emission intensity among the light emitting element arrays.
This correction is made by a following method. After a variation of the emission intensity when a light emitting element array wired and mounted on an optical printer head is driven by a constant voltage is measured as an initial value, drive voltages (currents) for the respective light emitting elements are so adjusted as to make the emission intensity uniform in the light emitting element arrays and among the light emitting element arrays. There are two such correcting methods.
A first correcting method is for adjusting values of currents flowing into the light emitting elements when the constant voltage is applied by adjusting (trimming) resistance components connected in series with the light emitting elements according to the initial value.
A second correcting method is for adjusting outputs of transistors forming the driver elements by correction data superposed on an image data according to the initial value.
FIG. 24 shows a block diagram of a driver circuit for making 16-stage corrections by the second correcting method. At least four transistors having different outputs are required per dot of the light emitting element, and at least four stages of correcting circuits 126 formed by a shift register having as many parallel outputs as the light emitting elements to be driven are required in order to input the correction data to the respective transistors and a latch.
In the case that the first correcting method is applied to the third prior art, a resistance value of the diffused resistor 108 shown in FIG. 22 needs to be adjusted. However, it is difficult to adjust the diffused resistance value by adjusting the density and depth of dopants diffusion and by annealing after the light emitting element array is wired and mounted on the optical printer head. Thin film resistance wiring portion whose resistance value can be trimmed or an other trimming circuit needs to be monolithically formed in addition to the diffused resistors.
In the case that the second correcting method is applied to the third prior art, a plurality of stages of shift registers and latches corresponding to the respective gradations need to be monolithically formed.
Accordingly, the chip size of the third conventional light emitting element array becomes larger as the size of the driver circuits becomes larger with the increase of their functions. Thus, a smaller number of chips can be obtained from one wafer.
Further, in proportion to the size of the monolithically formed driver circuits, a yield of the light emitting element arrays decreases. This also applies to the second conventional light emitting element arrays. The aforementioned problems residing in the second and third conventional light emitting element arrays cancel the reduced production cost and the improved reliability of the optical printer head which are obtained from the improved yield by considerably reducing the number of connection terminals and the reduced production cost by reducing the number of driver chips. Thus, the second and third conventional light emitting element arrays are a little realizable in view of the production cost.
It is an object of the present invention to provide a light emitting element, an optical printer head, and a method for driving an optical printer head which are free from the above-mentioned problems residing in the prior art.
It is an object of the present invention to provide a light emitting element which can easily make the light intensity variation of light emitting elements smaller, being miniaturized and being produced at a reduced cost, and an optical printer head using such light emitting element arrays, and a method for driving such an optical printer head.
A first aspect of the present invention is directed to a light emitting element array comprising a plurality of light emitting elements arranged in a row; switching elements connected in series with the respective light emitting elements and provided with control terminals for supplying emission signals to the respective light emitting elements; and an electrical driver for individually turning the respective switching elements on by individually supplying connection drive signals to the control terminals of the respective switching elements. The plurality of light emitting elements, switching elements, and the electrical driver are integrally formed in a semiconductor substrate.
In the light emitting element array thus constructed, since the switching elements connected in series are individually turned on, the lees connected therewith can be individually driven. Thus, when the switching elements are turned on, the emission signals, whose levels are so changed as to correspond to the respective light emitting elements, are supplied to the light emitting elements connected with these switching elements, thereby enabling an easy correction of a light intensity variation of the respective light emitting elements and an easy gradation control required for higher quality printing. Further, since the entire circuit construction is simpler as compared with the conventional ones, the light emitting element array can be made smaller and the production cost thereof can be reduced.
A second aspect of the present invention is directed to a light emitting element array comprising a plurality of light emitting elements arranged in a row and divided into a plurality of drive groups; switching elements connected in series with the respective light emitting elements of the respective drive groups, divided into a plurality of drive groups corresponding to those of the light emitting elements, and provided with control terminals for supplying emission signals to the respective light emitting elements; an electrical driver divided into a plurality of drive groups corresponding to those of the light emitting elements and adapted to individually turn the switching elements of the respective groups on at the same timings in each group by individually supplying connection drive signals to the control terminals of the switching elements of the respective drive groups in each drive group. The plurality of light emitting elements, the switching elements, and the electrical driver are integrally formed in a semiconductor substrate.
In the light emitting element array thus constructed, since the switching elements of the respective drive groups which are connected in series are individually turned on at the same timings in each drive group, the lees connected with these switching elements can be driven at the same timings in each drive group. Thus, when the switching elements of the respective groups are turned on, the emission signals, whose levels are so changed as to correspond to the respective light emitting elements, are supplied to the light emitting elements connected with these switching elements, thereby enabling an easy correction of the light intensity variation of the respective light emitting elements and an easy gradation control necessary for higher quality printing.
Further, since the entire circuit construction is simpler as compared with the conventional ones, the light emitting element array can be made smaller and the production cost thereof can be reduced. Furthermore, since the light emitting elements of the respective drive groups are driven in parallel at the same timings in each drive group, the light emitting elements can be driven at a higher speed, with the result that an optical printer head using these light emitting element arrays can print at a higher speed.
A third aspect of the present invention is directed to an optical printer head comprising a light emitting element array including a plurality of light emitting elements arranged in a row, switching elements connected in series with the respective light emitting elements and provided with control terminals for supplying emission signals to the respective light emitting elements and, and an electrical driver for individually turning the respective switcher on by individually supplying connection drive signals to the control terminals of the respective switching elements; and an emission controller for driving the electrical driver by supplying the connection drive signals thereto and individually driving the light emitting elements connected with the switching elements by supplying the emission signals thereto when these switching elements are individually turned on.
In the optical printer head thus constructed, the switching elements connected in series are individually turned on, and the lees connected with the turned-on switching elements are individually driven by supplying the emission signals thereof. Thus, when the switching elements are turned on, the emission signals, whose levels are so changed as to correspond to the respective light emitting elements, are supplied to the light emitting elements connected with these switching elements, thereby enabling an easy correction of the light intensity variation of the respective light emitting elements and an easy gradation control necessary for higher quality printing. Further, since the light emitting element array can be made smaller and the production cost thereof can be reduced, the optical printer head can be made smaller and the production cost thereof can be reduced.
A fourth aspect of the present invention is directed to an optical printer head comprising a plurality of light emitting elements arranged in a row and divided into a plurality of drive groups; switching elements connected in series with the respective light emitting elements of the respective drive groups, divided into a plurality of drive groups corresponding to those of the light emitting elements, and provided with control terminals for supplying emission signals to the respective light emitting elements; an electrical driver divided into a plurality of drive groups corresponding to those of the light emitting elements and adapted to individually turn the switching elements of the respective groups on at the same timings in each group by individually supplying connection drive signals to the control terminals of the switching elements of the respective drive groups in each drive group, and an emission controller for driving the electrical driver divided into the plurality of drive groups at the same timings in each drive group by supplying the connection drive signals to the electrical driver at the same timings in each group, and individually driving the light emitting elements of each drive group connected with the switching elements by supplying the emission signals thereto when these switching elements are individually driven at the same timings in each drive group.
In the inventive optical printer head thus constructed, the switching elements of the respective drive groups connected in series are individually turned on at the same timings in each drive group, and the lees connected with the turned-on switching elements are individually driven at the same timings in each group by supplying the emission signals thereto. Thus, when the switching elements of the respective groups are turned on, the emission signals, whose levels are so changed as to correspond to the respective light emitting elements, are supplied to the light emitting elements connected with these switching elements, thereby enabling an easy correction of the light intensity variation of the respective light emitting elements and an easy gradation control necessary for higher quality printing.
Further, since the light emitting element array can be made smaller and the production cost thereof can be reduced, the optical printer head can be made smaller and the production cost thereof can be reduced. Furthermore, since the light emitting elements of the respective drive groups are driven in parallel at the same timings in each drive group, the optical printer head can print at a higher speed.
A fifth aspect of the present invention is directed to a method for driving an optical printer head provided with a plurality of light emitting elements arranged in a row, comprising the steps of individually turning on a plurality of switching elements connected in series with the plurality of light emitting elements; and individually adjusting amounts of light emitted from the light emitting elements by supplying emission signals, whose levels are so changed as to correspond to the respective light emitting elements, to the light emitting elements connected with the switching elements when these switching elements are individually turned on.
According to this method, the light intensity variation of the respective lees can be easily corrected and the gradation control necessary for higher quality printing can be easily performed by a simple method. Further, since the lee array can be made smaller and the production cost thereof can be reduced by adopting this driving method, the optical printer head can be made smaller and the production cost thereof can be reduced.
These and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawings.